(1) Three weeks ago, I was working on a business project for five days.

(2) John was scoring a lot of goals for many years.

Some of my colleagues are native English speakers who have no English teaching experience. They think that there is a general grammar rule that requires you to use the simple past continuous tense if the word "for" is used in the sentence.

I have not been able to find such a rule in any online grammar sources. So, I am not sure if they are correct about the general rule. Are my sentences above grammatically correct? Please give me your feedback. Thank you very much for your help.